Angle modulation detector



S. W. SEE LEY June 13, 1950 ANGLE MODULATION DETECTOR Filed March 21, 1946 INVENTOR lzzarllf ATTORN EY Patented June 13, 195i) 2,511,448 Y ANGLE MODULATION DETECTOR Stuart W. Seeley, Roslyn Heights, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application March 21, 1946, Serial No. 655,989

My present invention relates generally to detectors of angle modulated carrier waves, and more particularly to a novel circuit for deriving the modulation signal from a frequency modulated (FM), or phase modulated (PM), carrier wave with minimum noise reproduction.

By angle modulation is meant either FM or PM, or hybrid forms of modulation possessing characteristics common to both of them. In the generation, transmission and reception of angle modulated Waves undesired amplitude modulation (AM) effects may come from the transmitter directly, may be due to interfering impulses, or may be caused by lack of uniform gain over the signal selector pass band at the receiver.

It is an important object of my present invention to provide a balanced FM detector which tends to keep itself apparently centered on a slowly shifting carrier frequency, such as for example during tuning or drifts in the transmitter or receiver frequencies, but acts as a normal balanced detector for rapid (audio) modulation of the carrier thereby permitting the balanced detector to operate at the center or balance point with reference to the signals with the result that the detector is least sensitive to noise impulses. v

Another object of my present invention is to provide a balanced detector composed of a pair of grid-controlled electron discharge devices provided with a common frequency discriminator input circuit, the devices being connected to provide a pair of rectiiiers in opposed relation relative to a common output circuit, and the respective control grids being bypassed back to their own cathodes for modulation frequencies,

5 claims. (ol. 25o-27) lowing description, taken in connection with the drawing, in which 1 have indicated diagrammatically a circuit whereby my invention may be carried into effect.

Referring now to the accompanying drawing, there is shown the modulation detector network of an FM receiver of the superheterodyne type. The detector is constructed in accordance with vmy present invention. While my invention is its mean frequency to an extent proportional to the amplitude of the modulations and independent of the modulating frequency. A PM wave differs in having a frequency deviation which increases with modulating frequency. The generic yexpression, angle modulation is, also, intended to include a modulated wave of preferably constant amplitude wherein the modulation is neither pure FM nor pure PM, but contains components resembling one or both of them, and is,

,ff-:Land that each transmitter radiates an FM wave but being cross-connected to opposite cathodes Y for direct current voltages to provide automatic centering action.V

Another object of my invention is to provide a pair of triodes in a balanced FM detector system; the anode to cathode paths of the triodes having a maximum frequency deviation up to 75 kilocycles (kc.) with respect to the normal transmitter frequency. These are the newly assigned frequency values of the FM broadcast band and are used herein merely by way of illustrabeing connected as opposed diode rectiers; the

control grids riding along in potential with their own respective cathodes for fast changes, as when frequency-representative modulation is applied to the grids, but a negative directv current voltage being applied to the grid of the triode getting the most FM signal in response to the receiver being tuned to one side of center frequency whereby the current outputs of the triodes tend to remainequal.

Another object of my invention is to provide an FM receiver which inhibits AM detection at the side of the center frequency of the desired FMsignals and. thereby enables the receiver to tune morev quietly, the inhibiting action all taking placeat the balanced FM detector circuit.

Othemobjects and-features of theinvention tion. The receiver may include any desired form of signal collector, as for example, a dipole. The collected FM signalwaves may be applied to a suitable converter for reduction of the mean fre-` quency value without change of the deviation. The-converter may be of any desired construction, and is preferably preceded by one or more stages of selective high frequency amplification. Suitable signal selector circuits, usually employing a variable condenser or adjustable inductor, are employed for adjustment to receive signals from a desired FM station. The signal selector circuits will, of course, preferably be adjusted accurately to resonate the various adjustable selector circuits to the center or mean frequency of the desired FM station.

In a superheterodyne receiver the converter is fed with oscillations from a local oscillator whose tank circuit includes an adjustable reactance device, usually a variable condenser or adjustable inductor. The latter is customarily adjusted concurrently with the aforesaid selector devices so that the tank circuit will be tuned to a local oscillation frequency d iiering, from the desired carrier i'qln'y by iihe`operating intermediate frequency '(IF). Theixselective circuits of, and preceding, the converter may on the other hand. be of the xedly tuned type, if desired. The in# termediate frequency is usually chosen. from 'a range of 2 to 20 mc., by way o fq present example 10.7 mc. Any suitable actua,tin",f,`f `V `r echemism ayV be used for operating the station selecting' d- Vices. The converter may use the wellfknovvn pentagrid tube, or it may useseparat oscillator and mixer tubes. These various circuits andencuit components are ver-y well known to'those skilled in the art of radio need oni'` be "lorieflyi i jmd to.

"Tlilf "Fainpliiier ne'tworkmayembody one or more am'pl'ier'tubes slectivenlyftuned to the illustratiye operating I. F. value "df` 1'0'7- mc. Oi coifirA all si AVtrank/Silo*ission circuitsy between the signal Acollector the demodulator or de tectoiwi-l-be's dconstri'icted as to pass lefliciently a 'beiiidA` 'at'l least,"V 150" kc.A widef It is,V also, usual to de'si n'th'fsignal'transmission circuits to have anlass bandofaperoxihtn 2Go kc. inlwidth to provide 1iii'r'`"reasonable4 tolerances, such as voscillat'r'fieduenc'y d'rftrandfthe like. The output tr'fansfois er' feeding" the final I. F. amplier tube us'flly h'asf' its primary and y secondary circuits ea'jl'tne'd Yto the one tingl. Evalue.

"Oneioff'th reasons 'iiifthe rpastfor employing an limitepr'iortovthediscriminator input lati'ng network) of the defv carrier W Wastavcid thenecessityfor critical toftlfe detectrfcircuit, since the demewdiat'ely preceding; the detector circuit may possess normal andffull gain,wlwiich is the reverse oi" the'isal'o: er'ating'condition for an amplitude ilf-ni discriminator input network ofV my present Fltldetector con? es vcoupled primary andv secortary 'circuits de tedbyvlnmerals 5 and I2 respective "Ilieinput'coil S indicated as part of vftle''ipri'n'iary` circuit.' While any known and SuitabT A( T A imary coil VIIl"is.hunted by condenser I5 to provide a parallel resonant circuit tuned to the operatiiig'i". F.""T'h"secoriaryvcqil I6 iscoupled l to primary coil I0 as indicated by numeral I1, and

coill I6` is Shnnted by condenser I 8. Thresonant secondaryfciricuit,IZQUincludin j coil I and oo ndenser Ii, ,isv tun'edto substan lly the resonant fread-,ener .Ofiihf .Precari Cir, s.

.communication and.,

sired AM effects on the'V .intsiiilay ble, utilized@ profi/ide. gnalficlises ,for .deteigrwbes I' 'prefer t'o explain the present circuit '1938. In', general', itisde'sired to eml 4 coils IQ and I6 may be of the known inductance trimmer type, or capacity tuning may be used. Specially, iron Vcores onslugsmay be. used for adjusting the ipdu'ctanceyalpies; offvv the. respective 5 coils Ii and I6, if coil I6 is so arranged that varying the slug does not unbalance the two halves ofthe coil. The high alternating potential side of coil I is connected by a lead to midpoint I6 of coil I. thus establishing the midpoint at the 'alternating potential as the high potential stibfunangenem 5. Detectors :a and I4 are shown, by Way of specific example, as electronidi'sch'ar'ge devices of the triode type. It is tozo'e` understood that the triodes may have their electrodes embodied in agcommon tube envelope. .2d andI located, in .the

pass.

' I' `re conglectelY*into,Y

d '.functicns,

ctor, l Itis topbemunderstoodgthat ventional FM detector circuit of the type emfploying balanced diodes. The modulation voltage, in this case the desired audio frequency modulation signal voltage, is taken off by connecting lead 28 to the upper end of load resistor R1. Condenser 29 is an audio frequency coupling condenser, and is inserted in the lead 28 to the input grid of the following audio frequency amplifier tube (not shown). Of course, one or more audio amplifier tubes may be employed, and the amplified audio frequency signals may be reproduced in any suitable manner, as by a loud speaker.

Before describing the electrical relations existing in the present FM detector circuit, there will generally be explained the manner in which the discriminator input network of the detector functions, reference specifically being made to my aforesaid patent. It is first assumed that the FM signals applied to the primary circuit are instantaneously at the mean or carrier frequency of 10.7 mc. The primary signal energies applied to the two diodes 22, and 2|, 23 will be of like phase. However, anode 22 and anode 2| are connected to opposite ends of secondary coil I6. Due to the coupling |'l between tuned circuits 5 and I2 there will be a 90 degree phase shift between the primary and the secondary circuit voltages, when the instantaneous carrier frequency at coil S is at the resonant or center frequency value.

Accordingly, the secondary signal voltage will be applied to anode 22 and anode 2| from the I respective ends of coil I6 in opposite phase, but

in each case in phase quadrature with the primary signal voltage. It follows, therefore, that the resultant signal voltages applied to anode 22 and anode 2| will be equal at the carrier frequency, and the rectified voltages across resistor R1 and R2 will be of equal magnitude.

If, at some later instant, the FM signals at circuit I have a frequency different from the resonant frequency of circuit I2, there will occur a phase shift of the signal energy transmitted through the transformer I0, I6 which is greater or less than 90 degrees, depending on the direction and the extent of frequency difference between the instantaneous frequency of the FM signals and the predetermined resonant frequency of the tuned circuits 5 and I2. This means that there will be applied to the diodes 22, 2|) and 2|, 23 resultant signal voltages of different magnitudes, and, therefore, the rectified voltages across respective loads R1 and R2 will be of different magnitudes.

It will now be appreciated that the detector tubes function to provide balanced FM detection in accordance with my aforesaid patent. By connecting each control grid to its respective cathode for audio frequency variations, and to the cathode of the opposite triode for direct current voltage variations, desirable characteristics are secured. If, for example, the receiver is mistuned from center frequency so that there tends to be more current fiow through one diode 22, 20 than the other diode 2|, 23, there will be a negative grid bias applied to the triode whose anode receives the larger vector sum voltage. Hence, that particular triode will have its rectiiication eiciency substantially reduced thereby causing equalization of the output currents of the opposed tubes.

For fast changes, as for frequency-representative modulation, the grids 50 and 5I literally ride along with their own respective cathodes 20 and 23, because of the audio bypass condensers 10 and 1|. The reason for connecting the grids to respective cathodes, then, for audio currents is to prevent the automatic centering mechanism from following the audio modulation. Hence, for the modulation voltage variations each grid and associated cathode act as a common electrode. However, if the FM receiver is tuned to one side of the desired FM carrier, or station, frequency, as when tuning through a channel or when the receiver is mistuned or detuned, a predetermined one of the grids will have its bias varied to an extent and in a sense to cause the current flow through each of resistors R1 and R2 tobe equal.

For example, assume a greater value of signal voltage is applied to tube I3 due to mistuning of the receiver. This will cause the current flow through load resistor R1 to exceed that through R2. Hence, optimum detection efficiency will not` exist, because the applied FM signals are not centered on the predetermined reference or center frequency of 10.7 mc. Noise will readily become apparent in such case, because balanced detectors of this type are least sensitive to noise impulses when they are accurately centered. Since, however, grid 50 is grounded through high resistance 60, which suppresses fast voltage variations, the cathode 20 will become positive and thereby effectively increase the negative bias of grid 50. This will tend to reduce the eiiiciency ofrectication of rectifier 22, 20 thereby equalizing the current fiow through resistors R1 and R2.

Conversely, greater signal voltage applied to tube I4, in response to opposite mistuning, will cause the voltage drop across resistor R2 to be larger, This will result in the grid 5| becoming more negative relative to grounded cathode 23.

Hence, the rectification efficiency of diode 2|, 23 will be reduced to such an extent that the output currents through resistors R1 and R2 are equalized. There is, therefore, provided automatic control of rectification eiiiciency to maintain the detector output current balanced. The triodes are, in effect, variable efficiency resistors, the eiiiciency being automatically adjusted by virtue of the balancing action of the cross-connecting grids so as to keep the detector outputs balanced even though the input signal is not accurately centered. There is provided an apparent selfcentering action in the detector circuit.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. In combination, a source of frequency modulation signals, a discriminator input circuit therefor, a pair of triode rectifiers, means connecting the anode of one rectifier and anode of the second rectifier to predetermined points of said discriminator input circuit, a separate condenser of low impedance to modulation frequency currents connecting the grid and cathode of each one of the triode rectifiers, a pair of series-connected load resistors lconnected between the cathodes, said discriminator circuit hfaving a connection from a point between said predetermined points to the junction of said pair of resistors, means for deriving modulation voltage from across said pair of resistors, and a direct current voltage cross-connection from each rec` 

